Some 6,000 troops are now posted along the border, and the armed forces have been tasked with arranging military accommodation for 5,000 unaccompanied minors.

Trump, who made stemming arrivals of undocumented migrants a plank of his 2016 election campaign, declared a national emergency on the border in February as a way to sidestep Congress for money to build a wall along the frontier.

"I'm going to work very closely with the acting secretary," acting Defense Secretary Patrick Shanahan said Wednesday.

He was referring to Kevin McAleenan, the replacement for Kirstjen Nielsen, who resigned over the weekend as director of homeland security.

"What I would say is, as the situation there deteriorates, it's pretty elastic in terms of demand on us so I would expect shortly here to have another request for assistance," he told reporters.

Shanahan has already released almost $1 billion to build a section of Trump's wall, cash diverted from the Pentagon budget for combating the drug trade.

The Pentagon announced Tuesday it had awarded two contracts worth $976 million for work on the proposed new wall and repairs to existing barriers in New Mexico and Arizona.

Shanahan said he did not rule out more military funds being assigned soon to wall construction.